Metamaterials are periodic materials having artificially fabricated inclusions in a host medium or on a host surface, that derive their properties, such as mechanical, optical and electrical properties, from the spatial distribution of the inclusions as well as from the properties of the subunits, as opposed to the properties of the components alone. Examples of man-made materials that do not exist in nature include sonic or phononic crystals (periodicity on the millimeter scale) and photonic crystals (periodicity on the sub-micrometer scale). Sonic crystals have a finite-sized periodic array of sonic scatterers embedded in a homogeneous host material and may have spectral gaps, which can be tuned by varying the size and geometry of the material, which prevent the transmission of sound waves having certain frequencies. If the host material is a solid, the term ‘phononic crystal’ is used for the artificial crystals, and both longitudinal and transverse shear waves may exist and may be coupled with one another. By contrast, for sonic crystals such waves are considered to be independent, and the scatterers are typically solid materials disposed in a fluid. A sonic crystal may be considered to be a sonic version of a photonic crystal, photonic crystals being periodic optical nanostructures having regularly repeating internal regions of high and low dielectric constant which affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons. Photons may be transmitted through such structures depending on their wavelength. Photonic and phononic effects occur when the spacing of the periodic structures is of the order of the wavelength of the photons or sound waves, respectively.
Photolithography and etching techniques similar to those used for integrated circuits have been used for fabricating three-dimensional photonic crystals. Photonic crystals have also been generated as self-assembled structures from colloidal crystals.